This invention relates to a threaded joint for steel pipes and particularly oil well pipes which has improved galling resistance, rust-preventing properties and gas tightness, and which can be used without the need to apply a liquid lubricating grease containing a heavy metal powder such as a compound grease.
Oil well pipes such as tubing and casing used in the drilling of oil wells are typically connected to each other by threaded joints. In the past, the depth of oil wells was typically 2000 to 3000 meters, but more recently, the depth of oil wells may reach 8,000 to 10,000 meters in offshore oil fields and other deep wells. A threaded joint for oil well pipes is subjected to various forces in its environment of use, including loads such as axial tensile forces caused by the weight of the oil well pipe and the threaded joint, the combination of internal and external pressures, and geothermal heat which increases as the depth increases. Therefore, a threaded joint needs to be able to maintain gas tightness without breakage in such environments.
During the process of lowering tubing or casing into a well, a joint which is once fastened for connection often needs to be loosened and then re-fastened. API (American Petroleum Institute) requires that there be no occurrence of galling, which is unrecoverable seizing, and that gas-tightness be maintained even if fastening (make-up) and loosening (break-out) are repeated ten times for a tubing joint and three times for a casing joint.
Oil well pipes are generally connected using a threaded joint with a pin-box structure having a metal to metal contact portion forming a good seal. In this threaded joint, typically, a pin is formed on the outer surface of each end of an oil well pipe, the pin having an externally threaded portion and a metal contact portion without threads. Such a metal contact portion is hereunder referred to as an xe2x80x9cunthreaded metal contact portionxe2x80x9d. A mating box is formed on the inner surface of a separate, sleeve-shaped coupling member, and it has an internally threaded portion and an unthreaded metal contact portion so as to mate with the corresponding threaded and unthreaded portions of the pin. When the pin is screwed into the box for fastening, the unthreaded metal contact portions of the pin and the box are made to tightly contact each other to form a metal seal.
In order to guarantee adequate sealing properties by the metal seal of a threaded joint in the environment of an oil well pipe, an extremely high surface pressure which may exceed the yield point of the material for the threaded joint must be applied to the unthreaded metal contact portions during fastening. This high pressure makes it easy for galling to take place. Therefore, prior to fastening, a lubricating grease called a compound grease, which is a viscous, flowable liquid, is applied to the metal contact portions and the threads in order to impart an increased galling resistance and gas tightness. Since the compound grease has good rust-preventing properties, it is frequently applied to a threaded joint before shipping the joint so as to prevent the joint from rusting before use in an oil well.
It has been proposed in the prior art to subject a threaded joint to surface treatment in order to improve the retention of a compound grease and impart decreased friction.
For example, Japanese Unexamined Patent Publication No. 61-79797 (1986) discloses a threaded joint having a threaded portion plated with Zn or Sn and a metal seal (unthreaded metal contact portion) plated with gold or platinum. Japanese Examined Patent Publication No. 3-78517 (1991) discloses a pipe joint having a coating of a synthetic resin which contains 20-90% of molybdenum disulfide having a particle diameter of 10 micrometers or less dispersed therein. Japanese Unexamined Patent Publication No. 8-103724 (1996) discloses a method for the surface treatment of a steel pipe joint comprising forming a lower manganese phosphate chemical conversion coating and an upper molybdenum disulfide-containing resin coating. Japanese Unexamined Patent Publication No. 8-105582 (1996) discloses a method for the surface treatment of a pipe joint comprising forming a first nitride layer, a second iron or iron alloy plated layer, and a third, uppermost molybdenum disulfide-containing resin coating.
Japanese Unexamined Patent Publication No. 62-258283 (1987) discloses a joint for oil well pipes which has a porous plated coating of zinc or a zinc alloy at least on the surface of its metal seal portion, the coating being formed by blast plating which is performed by blasting with particles having a core of iron or an iron alloy coated with a zinc or zinc alloy layer. It is described therein that the plated coating does not suffer from delayed fracture caused by hydrogen embrittlement and has good retention of a lubricant due to its porous nature, resulting in the advantage that gas tightness and galling resistance can be attained even with oil well pipes of a high-strength, high alloy steel. Thus, the coating formed by blast plating is contemplated using a compound grease as a lubricant.
All the threaded joints disclosed in the above-described patent publications are supposed to use with a compound grease to assure sufficient resistance to galling. However, the use of a compound grease involves problems with respect to the environment and gas tightness at high temperature as described below.
A compound grease contains a large amount of a powder of a heavy metal such as lead, zinc, or copper. Since a part of a compound grease applied to a threaded joint is discharged into the surrounding environment by washing off or by running out during fastening, the use of a compound grease raises a concern of causing environmental pollution. In addition, the application of compound grease worsens the work environment.
Recently, high-temperature oil wells having a temperature as high as 250-300xc2x0 C. due to an increased depth, as well as steam-injected oil wells into which steam at a high temperature close to the critical temperature (e.g., around 350xc2x0 C.) is injected in order to improve oil recovery have increased. Threaded joints for oil wells which are used in such a high-temperature environment are required to have a good heat resistance, as evidenced by maintaining gas tightness in a test where a joint which has been fastened is kept at a temperature above 350xc2x0 C. for a certain period and then subjected to loosening and re-fastening. With a compound grease, the lubricity may decrease at such a high temperature due to evaporation of one or more constituents of the grease, thereby leading to a failure to attain a desired gas tightness when the loosened joint is re-fastened in the test.
In order to eliminate the need to apply a compound grease, a threaded joint having a solid lubricating coating has been proposed. For example, a threaded joint having a solid lubricating coating which comprises a resin such as an epoxy resin having molybdenum disulfide as a solid lubricant dispersed therein is disclosed in Japanese Unexamined Patent Publications Nos. 8-233163 (1996), 8-233164 (1996), and 9-72467 (1997).
However, such a solid lubricating coating is inferior to a compound grease with respect to ability to protect a threaded joint from rusting under field conditions from shipment of the threaded joint to the use thereof in an oil well. Therefore, the threaded joint is apt to rust and thus form blisters or cause debonding in the solid lubricating coating, which in turn makes fastening of the joint unstable, leading to occurrence of galling or a decrease in gas tightness.
An object of the present invention is to provide a threaded joint for steel pipes having improved galling resistance, gas tightness, and rust-preventing properties, which can assure gas tightness in its metal seal portion after being subjected to fastening and loosening repeatedly without application of a heavy metal powder-containing liquid lubricating grease such as a compound grease.
Another object of the present invention is to provide a threaded joint for steel pipes having improved galling resistance, gas tightness, and rust-preventing properties, which can assure gas tightness in its metal seal portion after being subjected to fastening and loosening repeatedly even if it is used to fasten oil well pipes placed in a high temperature environment such as encountered in a deep, high-temperature oil well or a steam-injected oil well and/or even if it is made of any of a wide variety of steels including from a carbon steel to a high alloy steel.
The present inventors have found that these objects can be achieved by forming, on the surface of a threaded portion and an unthreaded metal contact portion of a threaded joint, a lower or undercoat layer of a porous zinc or zinc alloy coating such as formed by impact plating, e.g., such as blast plating and an upper solid lubricating coating or liquid, heavy metal powder-free lubricating coating.
Although it is not intended to be bound by any theory, the mechanism for this is considered to be as follows.
A blast-plated coating of zinc or a zinc alloy can be formed on the surface of a threaded joint for steel pipes irrespective of the steel material for the joint which encompasses from a carbon steel to a high alloy steel containing more than 13 wt % Cr by employing, as a blasting material, particles having an iron-based core the surface of which is coated with zinc or a zinc alloy as disclosed in Japanese Examined Patent Publication No. 59-9312 (1984).
Unlike an electroplated coating, the resulting blast-plated coating is porous. Therefore, when a solid or liquid lubricating coating is formed thereon, the lower blast-plated coating can improve adhesion of the solid lubricating coating or retention of the liquid lubricating coating. As a result, it is possible to exploit the lubricating coating to the best of its performance and thus attain improved galling resistance and gas tightness without the use of a compound grease.
The zinc present in the lower layer is a base metal and it ionizes in preference to iron constituting the substrate threaded joint and thus exhibits a sacrificial protection effect to prevent corrosion of the joint. However, because the porous coating of the lower layer includes numerous voids and has a large surface area, the sacrificial protection effect of this layer tends to be lost soon if the lower layer is not treated anymore. The formation of the overlaid lubricating coating serves to seal the voids of the lower layer and allows the lower layer to have a sustained protection effect to achieve improved rust-preventing properties. In particular, when the overlaid coating is a solid lubricating coating, a still greater improvement in galling resistance, rust-preventing properties, and gas tightness can be attained, thereby making it possible to provide a threaded joint having improved performance even in a high temperature environment.
Thus, in order to improve galling resistance, rust-preventing properties, and gas tightness, it is important that a liquid or preferably solid lubricating coating is formed on a lower porous zinc or zinc alloy layer.
The present invention is a threaded joint for steel pipes comprising a pin and a box capable of mating with each other, the pin having an externally threaded portion and an unthreaded metal contact portion as a mating surface, and the box having an internally threaded portion and an unthreaded metal contact portion as a mating surface, characterized in that the mating surface of at least one of the pin and the box is coated with a lower porous zinc or zinc alloy layer and an upper lubricating coating selected from a solid lubricating coating and a liquid, heavy metal powder-free coating.
A threaded joint according to an embodiment of the present invention has the porous zinc or zinc alloy layer and the overlaid lubricating coating on the mating surface of only one of the pin and the box. In this embodiment, the other mating surface may be either uncoated or coated with one or more coatings selected from a porous zinc or zinc alloy layer, a metal plated coating, a phosphate coating, an oxalate coating, and a borate coating.